Referring to FIG. 1, a portion of a typical video frame 10 is shown. The frame 10 includes alternating lines from two separate fields. The separate fields are generally referred to as even/odd, top/bottom, or field1/field2. The vertical resolution of each field is half that of the total video frame. When displayed n a NTSC standard television monitor, each field is shown for one-sixtieth of a second. Due to a characteristic of the human visual system known as persistence, the displayed fields appear to be consecutive and complete frames of motion video.
The frame 10 can be encoded using digital video compression for many applications. Transmission over limited bandwidth channels such as direct broadcast satellite (DBS) and storage on optical media (i.e., CD, DVD, etc.) are typical examples. In order to achieve efficient compression, complex, computationally intensive processes are used for encoding (or compressing) and decoding (or decompressing) digital video signals. Using conventional digital video compression, the frame 10 can be encoded via a standard such as MPEG-2.
The MPEG-2 compression standard operates on the basis of a variety of rules which eventually act to achieve a representation of the video sequence in a very optimized manner. The application of these specific rules and syntax to a video sequence creates a final stream of bits (i.e., a bitstream) that can be used to accurately replicate the pixels of the original frames of the source image.
MPEG-2 operates on a 16×16 pixel block basis. The 16×16 block is usually referred to as a macroblock. Macroblocks can have rows (or slices) representing interleaved field lines (e.g., the macroblocks 12 and 14). An MPEG-2 stream containing macroblocks with interleaved field lines is called a frame picture. Alternatively, the macroblocks can have rows representing information from a single field (e.g., the macroblocks 16 and 18). An MPEG-2 stream containing macroblocks with rows representing information from a single field is called a field picture. The MPEG-2 stream containing macroblocks with rows representing information from a single field occurs in the MPEG-2 syntax traditionally referred to as field picture mode. In the field picture mode, each field of the video frame 10 is encoded separately and converted into a bitstream. Support for field picture mode is mandatory according to the MPEG-2 decoder syntax.
During the encoding process, the video frame 10 can be coded in a fashion that complies with the syntax of the MPEG-2 video frame format, but actually transforms the source video frame to a different representation. One such transformation is when a video frame is comprised of alternating macroblock rows, with each row consisting of 16 vertical lines from each video field. Such a configuration may be advantageous for encoding and transmission of a particular set of video sequences. Additionally, the bitstream can be formed solely of intra-frame pictures. Although such a video frame format can be compressed, it is often desirable that the video sequence be decodable to the normal alternating even/odd field lines for presentation on a television monitor.
A solution for transforming an intra-only, frame picture encoded bitstream into a secondary format that can be decoded as interlaced field pictures by a standard, MPEG-2 compliant decoder would be desirable.